1. Field of the Invention
This invention relates generally to a process for recovering hydrocarbons from a subterranean formation having heterogeneous permeability, and in particular to a process for recovering hydrocarbons containing one or more volatile components from a heterogeneous subterranean formation
2. Description of Related Art
Most enhanced oil recovery processes were designed for use in subterranean formations having homogeneous permeability. These processes generally emphasize horizontal migration of fluids while maintaining horizontal fluid layers, commonly referred to as flow units, in the formation. In designing such processes, coning, or deflection of fluid interfaces, such as gas/oil or oil/water contacts, near production wells, has been viewed as a problem to be avoided. In accordance with one type of process, a gas, such as CO.sub.2, is injected into a subterranean formation and is dissolved in oil present therein to increase the oil volume and decrease the oil viscosity. Injected gas also is believed to replace oil in the formation matrix via a gravity drainage mechanism. Another type of enhanced recovery process involves heating the oil, thereby increasing the oil volume and decreasing the viscosity thereof. Thermal oil recovery processes have been used primarily, but not exclusively, with heavy oil which contains a very small fraction of volatile components. In some thermal recovery processes, distillation of volatile oil components is believed to contribute significantly to oil mobilization. Most thermal recovery processes have been conducted in relatively. unconsolidated sandstone formations. In another type of enhanced recovery process, the surface tension of the oil present in a subterranean formation is altered by flooding the formation with a surfactant, thereby promoting replacement of the oil in the formation matrix by the surfactant. In addition to increasing the quantity of oil recovered, these enhanced recovery processes, used singularly or in combination, may increase the rate of fluid movement from the formation matrix by a factor of about ten.
Enhanced oil recovery processes are generally ;less effective in formations with heterogeneous permeability distributions as, for example, in a highly fractured formation in which most of the oil is located in low-permeability matrix blocks which are surrounded by a high-permeability connected fracture network. It is generally believed that in such a heterogeneous formation, capillary forces trap a significant portion of the oil present in the low permeability blocks and inhibit oil production. Often, techniques have been employed to attempt to make the heterogeneous formation behave in a more homogeneous manner, rather than employing a process which takes advantage of the qualities of the heterogeneous formation.
U.S. Pat. Nos. 4,040,483 and 4,042,029 to J. Offeringa and SPE/DOE paper 20251 by J. N. M. van Wunnik and K. Wit describe processes in which a gas cap is created at the top of a heterogeneous-permeability formation to isolate oil bearing matrix blocks. Hot or cool gas is then injected into the reservoir to decrease the oil viscosity and increase the oil volume. Oil is also gravity replaced by gas that comes out of solution. All of these processes are believed to involve relatively slow gravity drainage of oil and focus upon overcoming Capillary forces to accelerate gravity drainage of liquid.
Thus, there is a need for a process that increases the quantity of relatively light, volatile liquid and gaseous hydrocarbon which can be recovered from a subterranean formation having heterogeneous permeability. An additional need is for a process to produce fluid from subterranean formations more rapidly.
Accordingly, a primary object of the present invention is to produce increased quantities of volatile fluid from a subterranean formation having heterogeneous permeability.
A further object of the present invention is to produce the fluid more rapidly.